This invention relates to an electron gun for a color picture tube, and in particular to the structure of electrodes constituting a main lens.
The spherical aberration of the main lens is enumerated among various factors, which influence remarkably the resolving characteristics of a color picture tube. It is known that the enlargement of the diameter of the electrodes constituting the main lens is efficacious for reducing the spherical aberration of the main lens.
However, for an inline electron gun since the circular cylindrical main lenses corresponding to three colors of R, G and B are arranged in one same horizontal plane, in a glass envelope, the diameter of the aperture should be smaller than 1/3 of the inner diameter of the neck portion containing the electron gun. By giving the thickness of the electrodes careful consideration and further by paying attention to the problematical points on the fabrication of the electrodes, this limit value is further decreased. If the inner diameter of the neck portion is increased, in order to raise this limit value, electric power for deflection increases. Moreover, in general, if the aperture is enlarged, the distance between the central axes of the beams becomes greater with increasing distance from the center of the beam which gives rise to worsening of convergence characteristics. Usually, since the diameter of the aperture is so designed that it is as large as possible while giving these points considerations, it is extremely difficult to further enlarge it.
U.S. Pat. No. 4,370,592 and 4,429,252 have proposed examples of non-circular cylindrical main lenses permitting effective enlargement of the aperture diameter beyond the above-mentioned limit value. The main lens disclosed in these patent specifications comprises two electrodes, each of which has an electrode plate provided with three apertures. Each of these electrode plates is disposed within a recess set back from the peripheral rim of the electrodes. Due to this arrangement, the electrostatic field produced by the counter electrode penetrates deeply into the interior of the electrode plate, gives rise to the same effect as the increase in the diameter of the aperture.
However, since the horizontal diameter of the cross-section of the peripheral portion of the electrode is greater than the vertical diameter thereof, the penetration of the electrostatic field is remarkable in the horizontal direction. Due to this fact, the focusing force of the lens in the horizontal direction is weaker than that in the vertical direction and thus astigmatism is produced in the electron beam. In order to correct this astigmatism, U.S. patent application Ser. No. 448,601 filed on Dec. 10, 1982, now U.S. Pat. No. 4,581,560, has proposed that the aperture is formed to be non-circular in such a manner that the aperture diameter in the horizontal direction is smaller than that in the vertical direction. In this way, the astignatism can be removed by strengthening the focusing electric field in the horizontal cross-section and by balancing the focusing forces in the horizontal and vertical directions.
When the angle of incidence of the electron beam to the electrode and the spread of the beam in the main lens are small, the astigmatism can be removed in this manner. However, when the beam spread is large, electrons pass in the neighborhood of the rim portion of the aperture in both the electrode plates. In this neighborhood, since the intensity of the electric field is high, the focusing force in the horizontal direction is stronger than that in the vertical direction. As the result, the point where electrons are focused in the horizontal direction is farther in front of the screen than that in the vertical direction. Consequently the diameter of the beam spot on the screen in the horizontal direction is greater than that in the vertical direction and thus the resolving power in the horizontal direction is reduced.
Further, these phenomena are more remarkable with decreasing distance between the axes of the beams. The reason therefor is the decrease of the horizontal diameter of the aperture. Therefore, there is a limit to the decrease of the distance between the axes of the beams for the purpose of ameliorating the convergence characteristics. When the diameter of the neck portion of the glass envelope is 29 mm, this limit is approximately 5.5 mm.